Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic method or an electrostatic recording method. In particular, the present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile, which includes a retaining member that retains a gap between an image bearing member and a developer bearing member.
Description of the Related Art
Conventionally, an image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic method includes a developing sleeve disposed in a position opposing a photosensitive drum. The photosensitive drum is an image bearing member on which an electrostatic latent image is formed, and the developing sleeve is a developer bearing member which carries and conveys the developer. The image forming apparatus then generally performs a developing process in which the electrostatic latent image is visualized using the developer.
An image forming apparatus employing a two-component developing method uses a two-component developer in which non-magnetic toner and magnetic carrier are mixed at a predetermined ratio. When such an image forming apparatus performs the developing process, the photosensitive drum and the developing sleeve are disposed opposing each other with a predetermined gap (i.e., an S-D gap) between each other. A developing bias is then applied to the developing sleeve, so that the developing process performed using the toner in the developer. In such a case, if the S-D gap is too wide, electric field intensity between the photosensitive drum and the developing sleeve is reduced, so that it becomes more difficult to perform the developing process. On the other hand, if the S-D gap is too narrow, the electric field intensity becomes excessively great, so that an abnormal electrical discharge becomes easily generated, and image defect such as ring-shaped dots may occur.
To solve such a problem, Japanese Patent Application Laid-Open No. 07-175321 discusses an image forming apparatus that retains the S-D gap within a predetermined range as follows. A gap retaining member is disposed at an edge portion of the developing sleeve to abut on the drum surface.
However, a problem arises in the image forming apparatus including an image forming unit that attaches the gap retaining member to the developing sleeve to abut on the photosensitive drum as discussed in Japanese Patent Application Laid-Open No. 07-175321. More specifically, there is friction against the gap retaining member due to rotation of the photosensitive drum or the developing sleeve. As a result, the gap retaining member becomes abraded, so that the S-D gap changes. In particular, abrasion is significant in the image forming apparatus in which a peripheral speed difference is set between the photosensitive drum and the developing sleeve that are disposed opposing to each other. The peripheral speed difference is set to convey a large amount of developer in a developing unit, i.e., the portion in which the photosensitive drum and the developing sleeve are disposed opposing each other, to improve developing performance. If the S-D gap becomes narrow due to abrasion of the gap retaining member, it becomes difficult for the two-component developer carried on and conveyed by the developing sleeve to pass through the S-D gap. Retention of the developer thus occurs in an area around the S-D gap.
The developer includes the magnetic carrier of which a thin layer is coated on the developing sleeve and is carried by a magnet disposed in the developing sleeve. If retention of the developer occurs in the area around the S-D gap as described above, the developer moves to a position in which the magnetic force of the magnet cannot hold the developer including the magnetic carrier. The developer including the magnetic carrier then overflows from a developing device, and eventually a transfer material to which the magnetic carrier attaches becomes discharged from the image forming apparatus. Further, the magnetic carrier that has overflowed from the developing device is conveyed to a transfer unit or a fixing unit along with the toner image, so that the image forming apparatus main body becomes greatly damaged.